Wis. Stat. § 971.15: Mental responsibility of defendant.

(1) A person is not responsible for criminal conduct if at the time of such conduct as a result of mental disease or defect the person lacked substantial capacity either to appreciate the wrongfulness of his or her conduct or conform his or her conduct to the requirements of law.

(2) As used in this chapter, the terms “mental disease or defect” do not include an abnormality manifested only by repeated criminal or otherwise antisocial conduct.

(3) Mental disease or defect excluding responsibility is an affirmative defense which the defendant must establish to a reasonable certainty by the greater weight of the credible evidence.